JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 38

laminates design
Through tensile strength data of coupons without holes (Figure 2), Stanford
University's team was able to cover the
laminate design variables of asymmetry and thick and thin plies. The lack
of sensitivity of tensile strength due to
mid-plane symmetry also means that
non-stop, continuous stacking from
the tool surface to the final layer can
be done in a single operation without
stopping at the mid-plane for a mirror
image of the second half of the laminate.
This non-stop stacking is faster and less
prone to error. The lack of mid-plane
symmetry is unorthodox and has not
been embraced by many designers. The
team produced several panels without
mid-plane symmetry. When the total
layers of bi-angle C-Ply reached 8,
there was no noticeable warpage after
curing. The data in Figure 2 confirmed
that the measured tensile strength of
these asymmetric laminates would not
be different from similar symmetric
laminates.
Equally important to laminate design
with or without mid-plane symmetry
is the degree of homogenization. With
a large number of layers, a laminate becomes more homogenized. The compar-

Curing insight

ison between the use of thick (268 gsm)
and thin (150 gsm) areal weight prepreg
was a measure of this homogenization.
The resulting strength was within the
scatter for smooth coupons, as shown in
Figure 2. This reduced repeating index
from 8T to 4T (for thin ply), and 4S to
2S (for thick ply) led to nearly the same
panel thickness between the use of thick
and thin layers. Again, there was no
noticeable difference in the measured
tensile strength. All the panels were
sufficiently homogenized for practical
purposes. When laminates are more homogenized, they automatically become
more symmetric, so that non-stop stacking can be used.
Figures 3 and 4 show the effect of loading the coupons parallel to one of the
plies [0°] or at a bisector angle of 22.5°
which keeps the fibres at the maximum
angle away from the loading direction.
The difference in unnotched strength is
evident in Figure 3 but with holes (Figure 4), there was no difference in tensile
strength due to the loading direction.
Figure 5 shows that the presence of
seams resulting from automatic tape
laying of C-Ply slightly lowers the

Conclusions
Laminate design can be freed from
many long-held rules so that lower cost
composite structures can be produced
with competitive if not superior
strength. With C-Ply, which can be engineered to offer multiple angle fabrics
for hand layup and multiple angle tapes
for automated tape laying machines,
a significant increase in productivity can be easily achieved. Moreover,
open hole coupons offer not only lower
failure load and closer tolerance but
also reflect a tolerance level for impact
damage and manufacturing defect. The
data presented in this paper show that
C-Ply is a competitive, safe and low
cost material. n

DEA for
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Cure monitoring of thermo-setting resins with Dielectric Analysis (DEA)
IDEX sensor in
lab furnace for
DEA 288 Epsilon

tensile strength of the laminates tested.
However, when tested with open holes
(which is what typical damage tolerant
structures are designed to), the seams
do not cause any decrease in tensile
strength even though in all coupons
with holes, the hole was drilled directly
through the outermost seam, producing
a "worst case" scenario.

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Formula development for resins
Cure optimization, e.g., during
composites manufacturing
Optimization of process parameters
Resin flow-front monitoring

NETZSCH-Gerätebau GmbH

38 jec composites magazine / No88 April 2014

More information:
www.stanford.edu
http://www.stanford.edu http://www.netzsch.com
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